U.S. patent number 7,469,883 [Application Number 11/497,025] was granted by the patent office on 2008-12-30 for cleaning method and solution for cleaning a wafer in a single wafer process.
This patent grant is currently assigned to Applied Materials, Inc.. Invention is credited to Kelly Truman, Steven Verhaverbeke.
United States Patent |
7,469,883 |
Verhaverbeke , et
al. |
December 30, 2008 |
Cleaning method and solution for cleaning a wafer in a single wafer
process
Abstract
The present invention is a novel cleaning method and a solution
for use in a single wafer cleaning process. According to the
present invention the cleaning solution comprises ammonium
hydroxide (NH.sub.4OH), hydrogen peroxide (H.sub.2O.sub.2), water
(H.sub.2O) and a chelating agent. In an embodiment of the present
invention the cleaning solution also contains a surfactant. And
still yet another embodiment of the present invention the cleaning
solution also comprises a dissolved gas such as H.sub.2. In a
particular embodiment of the present invention, this solution is
used by spraying or dispensing it on a spinning wafer.
Inventors: |
Verhaverbeke; Steven (San
Francisco, CA), Truman; Kelly (Morgan Hill, CA) |
Assignee: |
Applied Materials, Inc. (Santa
Clara, CA)
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Family
ID: |
26908694 |
Appl.
No.: |
11/497,025 |
Filed: |
July 31, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060270242 A1 |
Nov 30, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11145304 |
Jun 3, 2005 |
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09891730 |
Jun 25, 2001 |
6927176 |
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60214116 |
Jun 26, 2000 |
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Current U.S.
Class: |
261/102;
261/DIG.7; 261/105 |
Current CPC
Class: |
H01L
21/67051 (20130101); C11D 7/265 (20130101); C23G
5/00 (20130101); C11D 11/0047 (20130101); C11D
3/3947 (20130101); B08B 3/08 (20130101); C11D
3/32 (20130101); C11D 7/06 (20130101); C11D
3/044 (20130101); H01L 21/02052 (20130101); C11D
3/33 (20130101); C11D 1/72 (20130101); C11D
1/29 (20130101); Y10S 261/07 (20130101) |
Current International
Class: |
B01F
3/04 (20060101) |
Field of
Search: |
;261/100,102,104,105,107,DIG.7 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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496605 |
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Jul 1992 |
|
EP |
|
560324 |
|
Sep 1993 |
|
EP |
|
678571 |
|
Oct 1995 |
|
EP |
|
678571 |
|
Jan 1997 |
|
EP |
|
789071 |
|
Aug 1997 |
|
EP |
|
0860866 |
|
Feb 1998 |
|
EP |
|
926714 |
|
Jun 1999 |
|
EP |
|
1205968 |
|
May 2002 |
|
EP |
|
07060082 |
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Mar 1995 |
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JP |
|
WO 98/34579 |
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Aug 1998 |
|
WO |
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WO 02/01609 |
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Jan 2002 |
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WO |
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Other References
Clearly the Standard Online Newsletter, Pall Corporation,
Spring/Summer 2000. cited by examiner .
Naoaki et al., "Washing Method," Mar. 17, 1998, Computer-generated
English translation of JP 10-071375, 11 pages. cited by other .
Anderegg, Von G., et al., "Hydroxamatkomplexe III.sup.1). Eisen
(III)-Austausch zwischen Sideraminen und Komplexonen Diskussion der
Bildungskonstanten der Hydroxamatkomplexe," Helvetica Chimica Acta,
vol. XLVI, Fasciculus IV (1963)--No. 156, pp. 1409-1422, Basel 7
(Schweiz). cited by other .
Birus, Mladen, et al., "Iron (III) Complexation by Desferrioxamine
B in Acidic Aqueous Solutions. Kinetics and Mechanism of the
Formation and Hydrolysis of the Binuclear Complex Diferrioxamine B,
Inorganic Chemistry," vol. 23, No. 14, 1984, pp. 2170-2175,
.COPYRGT. 1984 American Chemistry Society. cited by other .
Birus, Mladen, et al., "Iron (III) Complexation by Desferrioxamine
B in Acidic Aqueous Solutions. The Formation of Binuclear Complex
Fiferrioxamine B," Inorganica Chimica Acta, vol. 78 (B6) N. 2, Feb.
1983, pp. 87-92, .COPYRGT.. Elsevier Sequoia/Printed in
Switzerland. cited by other .
Birus, Mladen, et al., Kinetics and Mechanism of Interactions
Between Iron (III) and Desferrioxamine B. The Formation and
Hydrolysis of Ferrioxamine B in Acidic Aqueous Solution, Croatica
Chemica Acta, CCACAA 56 (1) pp. 61-77, Apr. 2007. cited by other
.
Birus, Mladen, et al., "Kinetics of Stepwise Hydrolysis of
Ferrioxamine B and of Formation of Diferrioxamine B in Acid
Perchlorate Solution, Inorganic Chemistry," vol. 26, No. 7, 1987,
pp. 1000-1005, .COPYRGT. 1987 American Chemical Society. cited by
other .
Birus, Mladen, et al., "Mechanistic and Equilibrium Study of the
Iron (III) Complexation by Deferriferrioxamine B in Aqueous Acidic
Solution. Evidence for the Formation of Binuclear Diferrioxamine B,
Inorganica Chimica Acta," Bioinorganic Chemistry Articles And
Letters, vol. 56(B3), No. 2, Aug. 1981, pp. L43-L44, .COPYRGT.
Elsevier Sequoia S.A., Lausanne, Printed in Switzerland. cited by
other .
Evers, Ann, et al., "Metal Ion Recognition in Ligands with
Negatively Charged Oxygen Donor Groups. Complexation of Fe(III),
Ga(III), In(III), Al(III), and Other Highly Charged Metal Ions,
Inorganic Chemistry," vol. 28, No. 11, 1989, pp. 2189-2195,
.COPYRGT. 1989 American Chemical Society. cited by other .
Gould, Brian, et al., "A Thermodynamic Description of the Binding
of Iron to Ferrioxamine B in Aqueous Solutions, Archives Of
Biochemistry And Biophysics," vol. 215, No. 1, Apr. 1, 1982, pp.
148-156, .COPYRGT. 1982 by Academic Press, Inc., A Subsidiary of
Harcourt Brace Jovanovich, Publishers, New York, London, Paris, San
Diego, San Francisco, Sao Paolo, Sydney, Tokyo, Toronto. cited by
other .
Harju, Leo, "The Stability Constants Of Some Metal Chelates Of
Triethylenetetraminehexaacetic Acid (TTHA)," Analytica Chimica
Acta, vol. 50, 1970, pp. 475-489, Elsevier Publishing Company,
Amsterdam, Printed in The Netherlands. cited by other .
Harju, Leo, et al., "Titrations With Complexing Agents Forming
Mononuclear And Binuclear Complexes With Metals," Analytica Chimica
Acta, vol. 49, 1970, pp. 205-219, Elsevier Publishing Company,
Amsterdam, Printed in The Netherlands. cited by other .
Khan, M.M. Taqui, et al., "Aminopolycarboxylic Acid Complexes of
Al(III), Ga(III) & In(III)," Indian Journal of Chemistry, vol.
19A, Jan. 1980, pp. 50-57, published by The Council Of Scientific
& Industrial Research, New Delhi, India. cited by other .
Ma, Rong, et al., "Protonation constants and metal ion binding
constants of N,N',- bis
(2-hydroxyphenyl)-N,N'-ethylenediaminediacetic acid," Inorganica
Chimica Acta, The International Inorganic Chemistry Journal, vol.
209, No. 1, 1993, pp. 71-78, ..COPYRGT. 1993 Elsevier Sequoia.
cited by other .
Monzyk, Bruce, et al., "Kinetics and Mechanism of the Final Stage
of Ferrioxamine B Aquation in Aqueous Acid," Inorganica Chimica
Acta, Bioinorganic Chemistry Articles and Letters, vol. 55 (B2) No.
1 Jan. 1981, pp. L5-L7, .COPYRGT. Elsevier Sequoia S.A.,
Lausanne-Printed in Switzerland. cited by other .
Monzyk, Bruce, et al., "Kinetics and Mechanism of the Stepwise
Dissociation of Iron (III) from Ferrioxamine B in Aqueous Acid,"
Journal Of The American Chemical Society, vol. 104, No. 18, 1982,
pp. 4921-4929, .COPYRGT. 1982 American Chemical Society. cited by
other .
Ohman, Lars-Olof, "Equilibrium and Structural Studies of Silicon
(IV) and Aluminum (III) in Aqueous Solution. 21. A Potentiometric
and .sup.27 A1 NMR Study of the System
H.sup.+-A1.sup.3+-MoO.sub.4.sup.2-," Inorganic Chemistry, vol. 28,
No. 19, 1989, pp. 3629-3632, .COPYRGT. American Chemical Society.
cited by other .
Winston, Anthony, et al., "Hydroxamic Acid Polymers. Effect of
Structure on the Selective Chelation of Iron in Water,
Macromolecules," vol. 11, No. 3, May-Jun. 1978, pp. 597-603,
.COPYRGT. 1978 American Chemical Society. cited by other .
Yoshida, Isao, et al., New multidentate ligands. XXI. Synthesis,
proton, and metal ion binding affinities of
N,N',N'-tris[2-(N-hydroxycarbamoyl)ethyl]-1,3,5-benzenetricarboxamide
(BAMTPH), Canadian Journal of Chemistry, vol. 61, No. 12, Dec.
1983, pp. 2740-2744, National Research Council Canada, Printed in
Canada by K.G. Campbell Corporation. cited by other .
Schwarzenbach, Von G., et al., "Hydroxamatkomplexe I. Die
Stabilitat der eisen (III)-Komplexe einfacher Hydroxamsauren und
des Ferrioxamins B," Helvetica Chimica Acta., vol. XLVI, Fasciculus
IV, No. 154, 1963, pp. 1390-1400, Basel 7 (Schweiz). cited by other
.
Valtron Specialty Chemicals for Tomorrow's Technology, VALTRON DP
Series Formulated Detergents, 1 page, Mar. 2004. cited by other
.
"Chapter 3--Silicon Wafer Cleaning Procedure,"
http://www-mtl.mit.edu/CAFE/sop.sub.--copy/rca.html, Apr. 24, 2002,
4 pages. cited by other .
Patent Abstracts of Japan, vol. 004, No. 089 (C-016), Jun. 25, 1980
& JP 55 051427 A (Sakaoka Kazuhiko), Apr. 15, 1980, 1 page.
cited by other .
"Silicon VLSI Technology, Fundamentals, Practice and Modeling," By
Plummer, Deal and Griffin, IC Manufacturing-Chapter 4,
"Semiconductor Manufacturing-Clean Rooms, Wafer Cleaning And
Gettering"-Chapter 4, .COPYRGT. 2000 by Prentic Hall, Upper Saddle
River, N.J., 16 pages. cited by other .
Written Opinion for PCT/US 01/41160 mailed Apr. 2, 2004, 7 pages.
cited by other .
2nd Annual International SEMATECH Wafer Cleaning and Surface
Preparation Workshop 2000, Apr. 11-12, 2000, Hyatt Hotel, Austin,
TX, 24 pages. cited by other .
John F. Gibson et al., "Chelating Tendencies of N,N.sup.'-Bix
(2-hydroxyphenyl)-Ethylenediamine-N,N'-Diacetic Acid", J. Chem.
Soc. Dalton Trans, 1992, pp. 1375-1379, XP008000227. cited by other
.
Hitoshi Morinaga et al., "Advanced Alkali Cleaning Solution for
Simplification of Semiconductor Cleaning Process", Materials
Research Society Symposium Proceedings, Materials Research Society,
Pittsburg, PA, vol. 477, 1997, pp. 35-46, XP000978137. cited by
other .
International Search Reportand Written Opinion of the International
Searching Authority, for PCT/US2006/021503, mailed Dec. 20, 2006,
18 pages. cited by other .
"Carbonic acid," Wikipedia, the free encyclopedia,
http://en.wikipedia.org/wiki/Carbonic.sub.--acid; pp. 1-4, 2007.
cited by other.
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Primary Examiner: Bushey; Scott
Attorney, Agent or Firm: Blakely Sokoloff Taylor &
Zafman, LLP
Parent Case Text
This application is a divisional of U.S. patent application Ser.
No. 11/145,304, filed on Jun. 3, 2005, which is a divisional of
U.S. patent application Ser. No. 09/891,730 filed Jun. 25, 2001,
now U.S. Pat. No. 6,927,176, which claims the benefit of U.S.
Provisional Application No. 60/214,116, filed Jun. 26, 2000
entitled CLEANING METHOD AND SOLUTION FOR CLEANING A WAFER IN A
SINGLE WAFER PROCESS.
Claims
We claim:
1. A method of preparing a rinse solution comprising: using a
stacked membrane consisting essentially of a solid membrane on top
of a porous membrane; passing H.sub.2O along said porous membrane;
and passing CO.sub.2 gas along said solid membrane; and diffusing
said CO.sub.2 gas through said solid membrane and dissolving said
CO.sub.2 gas into said H.sub.2O.
2. The method of claim 1 wherein said CO.sub.2 gas contains
residual byproduct organic impurities from conversion of petroleum
to said CO.sub.2 gas, and said diffusing CO.sub.2 gas further
comprises diffusing said CO.sub.2 gas minus said organic impurities
through said solid membrane.
3. The method of claim 1 further comprising diffusing said CO.sub.2
gas through said solid membrane and dissolving said CO.sub.2 gas
into said H.sub.2O until said rinse solution has a resistivity less
than 5 megaohm.cm.
4. The method of claim 1 further comprising, passing said H.sub.2O
through a multiplicity of conduits formed between a multiplicity of
said stacked membranes arranged in parallel.
5. The method of claim 4, wherein said H.sub.2O passes along said
porous membranes of said multiplicity of stacked membranes.
6. A method of preparing a rinse solution comprising: using a first
stacked membrane comprising a first solid membrane in contact with
a first porous membrane to prepare a rinse solution, wherein said
preparation comprises: passing H.sub.2O along said first porous
membrane; passing CO.sub.2 gas along said first solid membrane; and
diffusing said CO.sub.2 gas through said first solid membrane and
dissolving said CO.sub.2 gas into said H.sub.2O until said rinse
solution has a resistivity less than 5 megaohm.cm.
7. The method of claim 6 wherein said CO.sub.2 gas contains
residual byproduct organic impurities from conversion of petroleum
to said CO.sub.2 gas, and said diffusing CO.sub.2 gas further
comprises diffusing said CO.sub.2 gas minus said residual byproduct
organic impurities through said first solid membrane.
8. The method of claim 6 wherein the first stacked membrane
consists essentially of a first solid membrane in contact with a
first porous membrane.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of semiconductor
processing and more specifically to a cleaning solution and a
method of using the cleaning solution in a single wafer cleaning
process.
2. Discussion of Related Art
Wet etching and wet cleaning of silicon wafers is usually done by
immersing silicon wafers into a liquid. This is also sometimes done
by spraying a liquid onto a batch of wafers. Wafer cleaning and
etching is traditionally done in a batch mode where several wafers
(e.g. 50-100 wafers) are processed simultaneously. A typical
cleaning sequence consists of HF-SC1-SC2. HF (HydroFluoric acid) is
a dilute HF solution used for etching thin layers of oxide. This is
typically followed by the Standard Clean 1 (SC1 solution) that
consists of a mixture of NH.sub.4OH, H.sub.2O.sub.2, and H.sub.2O.
Sometimes the SC1 solution is also called the APM solution, which
stands for Ammonia hydrogen Peroxide Mixture. The SC1 solution is
mainly used for removing particles and residual organic
contamination. The SC1 solution, however, leaves metallic
contaminants behind.
The final solution is the Standard Clean 2 solution (SC2) that is a
mixture of HCl, H.sub.2O.sub.2, and H.sub.2O. Sometimes the SC2
solution is also called the HPM solution, which stands for
Hydrochloric acid hydrogen Peroxide Mixture. The SC2 solution is
mainly used for removing metallic contamination. The particular
sequence of SC1 and SC2 is most often referred to as the RCA (Radio
Corporation of America) cleaning sequence. Between the HF, SC1, and
SC2 solutions there is usually a DI (de-ionized) water rinse. There
is usually a DI water rinse after the SC2 solution.
The total time for a standard clean cycle is on the order of 64-70
minutes as shown in FIG. 1a. The HF step takes approximately 1-5
minutes. The SC1 step typically takes 10 minutes and the SC2 step
also typically takes 10 minutes. The intermediate and final DI
water rinse steps take about 8-10 minutes. The final drying of the
wafers typically takes between 10-15 minutes. Typically 50-100
wafers are processed at the same time. If separate baths are used
for different chemicals then after one batch with 50-100 wafers
leaves a bath, a new batch 50-100 wafers can be loaded. Usually the
rate limiting step is the dryer which takes up to 15 min. This
means that roughly every 15 minutes a new batch of either 50-100
can be processed resulting in an overall throughput for the system
of between 200-400 wafers per hour, respectively for batches of 50
or 100 wafers.
Because there is a need for shorter cycle times in chip
manufacturing, there is a need for a fast single wafer cleaning
process. In order to make a single wafer cleaning process
economical, the processing time per wafer should be on the order of
two minutes. This means the entire HF-SC1-SC2 sequence, which
normally requires about 64-70 minutes, must be completed within two
minutes and at least within three minutes. Unfortunately, presently
it is not possible to perform an SC1-SC2 cleaning sequence in less
than two minutes and at least within three minutes. Until now, wet
processing is usually done in a batch mode, since the throughput of
single wafer processing cannot compete with batch processing.
Thus, what is desired is a method of reducing the SC1 and SC2
cleans from the normal processing time to less than or equal to
1-1/2 minutes. It is also required to reduce the time necessary for
the HF step and the dry. The present invention shows how to reduce
the time of the SC1 -SC2 sequence from roughly 40 min down to 1-1/2
min for use in a single wafer fashion and at least less than three
minutes for the entire cleaning cycle including HF, cleaning,
rinsing and drying.
SUMMARY OF THE INVENTION
The present invention is a method of use of a novel cleaning
solution in a single wafer cleaning process. According to the
present invention the method involves using a cleaning solution in
a single wafer mode and the cleaning solution comprises at least
ammonium hydroxide (NH.sub.4OH), hydrogen peroxide
(H.sub.2O.sub.2), water (H.sub.2O) and a chelating agent. In an
embodiment of the present invention the cleaning solution also
contains a surfactant. In another embodiment of the present
invention the cleaning solution also includes a dissolved gas such
as H.sub.2. The same cleaning solution containing ammonium
hydroxide, hydrogen peroxide, a chelating agent, and/or a
surfactant and/or dissolved hydrogen may also be used in a multiple
wafer mode for certain applications. The present invention is also
a DI water rinse solution that includes an oxidant and CO.sub.2
gas. All of these elements work in combination to improve
processing efficiency.
Moreover, the present invention also teaches a method of combining
an ammonia hydroxide, hydrogen peroxide, and chelating agent step
with a short HF step in a fashion that minimizes process time in a
way that the entire method removes aluminum and iron contamination
efficiently without etching too much oxide.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a time-line showing the traditional HF-SC1-SC2 wet bench
procedure for an oxide etch and a hydrophilic clean (RCA
clean.)
FIG. 1b is a time-line showing the cleaning process of the current
invention for an oxide etch and hydrophilic clean in a single wafer
cleaning tool.
FIG. 2a is a structure of a common chelating agent.
FIG. 2b is a structure of a common chelating agent that has bound
metal ions at its ligand sites.
FIGS. 3a-3d are structures of specific chelating agents that are
particularly useful in the current invention.
FIG. 4a is an illustration of an hydroxide terminated silicon
dioxide film.
FIG. 4b is an illustration of a silicon dioxide film terminated by
metal ions.
FIG. 5 is an illustration showing the surfactant attached to a
particle in solution and to the surface of a wafer.
FIG. 6a is an illustration of a cross-sectional view of a single
wafer cleaning apparatus.
FIG. 6b is an illustration showing the covering of the entire
surface area of a plate with transducers.
FIG. 6c is an illustration showing how the transducers covered
plate of FIG. 6b covers the entire surface area of a wafer being
cleaned.
FIG. 6d is an illustration showing a close-up the venturi device
that can be used in the single wafer cleaning apparatus.
FIG. 7a is an illustration of a membrane device that can be used in
the single wafer cleaning apparatus.
FIG. 7b is a cross sectional illustration of a modified membrane
that may be used in the membrane device of FIG. 7a.
FIG. 7c is an illustration of how the modified membrane works.
FIG. 8 is a flow-chart of the first embodiment of an HF etch and
cleaning process for use in a single wafer cleaning apparatus.
FIG. 9 is a flow-chart of the second embodiment of an HF etch and
cleaning process for use in a single wafer cleaning apparatus.
FIG. 10a is an illustration of a silicon wafer with an oxide layer
before an HF etching step.
FIG. 10b is an illustration of a silicon wafer with a hydrophobic
silicon surface after an HF etching step.
FIG. 11 is a flow-chart of a cleaning process in a single wafer
cleaning apparatus after an O.sub.2 ashing step.
FIG. 12 is a flow-chart of a cleaning process employing a short HF
etching step.
FIG. 13a is an illustration of the silicon dioxide film on a
silicon wafer before a short HF etching step and FIG. 13b is the
silicon wafer after a short HF etching step.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
In the following description numerous specific details are set
forth in order to provide a thorough understanding of the present
invention. One of ordinary skill in the art will understand that
these specific details are for illustrative purposes only and are
not intended to limit the scope of the present invention.
Additionally, in other instances, well-known processing techniques
and equipment have not been set forth in particular detail in order
to not unnecessarily obscure the present invention.
The present invention is a method, a solution, and a rinse for use
in a single wafer cleaning process. The method is specifically
useful for single wafer cleaning, but it may also be used in
applications where more than one wafer is cleaned at a time. The
novel cleaning solution is formulated in such a way as to increase
the efficiency of the cleaning process. Both the cleaning solution
and the rinsing solution are specifically useful for the removal of
ionic metallic impurities and particles during the front of the
line semiconductor processing sequence when the active regions of
the device are exposed.
The wafer cleaning solution of the present invention consists of
the solution resulting from the mixture of ammonium hydroxide
(NH.sub.4OH), hydrogen peroxide (H.sub.2O.sub.2), water (H.sub.2O),
a chelating agent, and a surfactant. As well known in the art these
compounds only dissociate into their respective ions and no
chemical reactions occur among these compounds. The ammonium
hydroxide (NH.sub.4OH), hydrogen peroxide (H.sub.2O.sub.2), and
water (H.sub.2O) are present in concentrations defined by dilution
ratios of between 5/1/1 to 1000/1/1, respectively. The ammonium
hydroxide/hydrogen peroxide ratio can also be varied between 0.05/1
and 5/1 and in some cases no hydrogen peroxide is used at all. The
ammonium hydroxide in this cleaning solution would be from a
solution of 28-29% w/w of NH.sub.3 to water. The hydrogen peroxide
in this cleaning solution would be from a solution of 31-32% w/w of
H.sub.2O.sub.2 to water.
The purpose of the ammonium hydroxide and the hydrogen peroxide in
the cleaning solution is to remove particles and residual organic
contaminates from a wafer that comprises a monocrystalline silicon
substrate on at least its front end. The purpose of the cleaning
solution is also to oxidize the surface of the wafer. According to
the preferred embodiment of the present invention the cleaning
solution has an alkaline pH level due to the ammonium hydroxide and
the hydrogen peroxide of between 9 and 12 and more specifically
between 10 and 11.
The purpose of the chelating agent is to remove metallic ions from
the wafer. Chelating agents are also known as complexing or
sequestering agents. These agents have negatively charged ions
called ligands that bind with free metal ions and form a combined
complex that will remain soluble. The ligands bind to the free
metal ions as follows: M.sup.x++L.sup.y-.fwdarw.M.sup.(x-y)+L This
is demonstrated in FIG. 2a and FIG. 2b with the common chelating
agent ethylenediaminetetraacetic acid (EDTA.) In FIG. 2a the EDTA
ion is not bound to any metal ions (M.sup.x+). In FIG. 2b it is
shown that one EDTA can bind up to six metal ions (M.sup.x+).
Common metallic ions that would be present on the wafer are copper,
iron, nickel, aluminum, calcium, magnesium, and zinc. Other
metallic ions may also be present.
Suitable chelating agents include polyacrylates, carbonates,
phosphonates, and gluconates. There are several specific chelating
agents that would be particularly useful as part of the cleaning
solution. They are: ethylenediaminetetraacetic acid (EDTA) (see
FIG. 2a), N,N'-bis(2-hydroxyphenyl)ethylenediiminodiacetic acid
(HPED) (see FIG. 3a), triethylenetetranitrilohexaacetic acid (TTHA)
(see FIG. 3b), desferriferrioxamin B (see FIG. 3c),
N,N',N''-tris[2-(N-hydroxycarbonyl)ethyl]-1,3,5-benzenetricarboxamide
(BAMTPH) (see FIG. 3d), and
ethylenediaminediorthohydroxyphenylacetic acid (EDDHA). These
chelating agents were chosen because they each had an equilibrium
constant (K) greater than 10.sup.15, and preferably greater than
10.sup.20 for Al.sup.3+. These K values were desired because they
mean that the chelating agent will be able to remove aluminum from
the wafer. The preferred range of concentrations for the chelating
agent is between 0.001 mg/l to 300 mg/l and more specifically
between 0.01 mg/l to 3 mg/l. Or alternately the chelating agent
should be between 1-400 ppm of the cleaning solution, and
preferably around 40 ppm of the cleaning solution. These
concentrations are suitable because they enable the reduction of
free metallic ions by roughly a factor of 10.sup.6 or higher
depending on the metallic ion.
During the modified SC1 clean, the surface of the wafer is covered
with a silicon dioxide film terminated by hydroxide groups (Si--OH)
as shown in FIG. 4a. Metals are bound to this surface as
(Si--O).sub.yM.sup.(x-y)+ as shown in FIG. 4b. The equilibrium
reaction governing the binding (chemisorption) and unbinding
(desorption) is described by the following equation:
M.sup.x++y(Si--OH).fwdarw.(Si--O).sub.yM.sup.(x-y)++yH.sup.+ From
this equation, one can see that there are two ways to remove
metallic ions from the oxide surface. The first way is to increase
the acidity [H+] of the solution. This produces a solution where
most of the metallic ions that are common in semiconductor
processing are soluble provided that there is a suitable oxidizing
agent in the solution. Suitable oxidizing agents include O.sub.2,
H.sub.2O.sub.2, and O.sub.3. The suitability of these ions is
determined by their ability to prevent the reduction of any ions in
the solution, such as copper (Cu.sup.2+.) Increasing the acidity
and having a suitable oxidizing agent present is the method used by
the most common metallic impurity removing solution, i.e. SC2.
The second way of removing metallic ions from the oxide surface is
to decrease the free metal ion concentration [M.sup.x+] in the
solution. The free metal ion concentration of the solution may be
decreased by adding a chelating agent to the solution. The same
level of metal ion impurity removal found through the use of the
SC2 solution may be achieved though the use of a chelating agent in
the SC1 solution (the modified SC1 solution) by meeting two
requirements. The first requirement is that the complex of the
chelating agent and the bound metal ion remains soluble. The second
requirement is that the chelating agent binds to all the metal ions
removed from the wafer surface.
Chelating agents may be added to the solution at two distinct
points. First, the agents may be added to the SC1 solution itself
before the solution is dispensed onto the silicon wafer. Second,
the agents may be added to concentrated NH.sub.4OH at the chemical
manufacturing plant and shipped as a mixture to the integrated
circuit manufacturer. Alternatively, the chelating agents may be
added to the H.sub.2O.sub.2 at the manufacturing plant. This,
however, is less desirable because many chelating agents are slowly
oxidized by H.sub.2O.sub.2.
The advantages of using chelating agents to remove metallic
impurities are that they do not require an acidic environment and
that they reduce the overall cleaning time. Other methods of
removing metal ions, such as the SC2 solution, require an acidic
environment. Traditionally, acidic environments were required for
the removal of metal ions therefore requiring that the metal ion
removal step be done separately from the SC1 step. This is because
the SC1 solution is very alkaline. Chelating agents work in very
alkaline environments, allowing them to be added to the SC1
solution. By combining the metal ion removal with the SC1 cleaning
step the overall cleaning time is reduced by eliminating the SC2
step. In the traditional SC1-SC2 cycle each step took about ten
minutes. Because this cycle is typically repeated many times in the
front end of semiconductor processing, the combination of the steps
will dramatically reduce the cleaning time.
In an alternate embodiment the cleaning solution contains a
surfactant. The purpose of the surfactant is to prevent
reattachment or redeposition of particles on the wafer after they
have been dislodged from the wafer. Preventing the reattachment of
the particles is important because allowing the particles to
reattach increases overall cleaning times. Therefore the surfactant
is used to reduce the cleaning time and to make single wafer
cleaning possible in less than two minutes as compared to 64
minutes in a batch type method (see FIG. 1a and FIG. 1b)
Surfactants are long hydrocarbon chains that typically contain a
hydrophilic (polar water soluble group) and a hydrophobic group (a
non-polar water insoluble group). The surfactants attach with their
non-polar group to particles 500 (FIG. 5) as well as to the surface
of the wafer 510. As a result the polar group of the surfactant 520
will point away from the wafer and away from the particles 500
towards the solution. Because of this the particles in the solution
that are bound by the surfactant will be repelled electrostatically
from the surface of the wafer because of the polar groups of the
surfactant on both the particles and the surface of the wafer as in
FIG. 5. The surfactant in the present invention is non-ionic,
anionic, or a mixture of non-ionic and anionic compounds. Non-ionic
means that the polar end of the surfactant has an electrostatic
rather than an ionic charge and anionic means that the polar end of
the surfactant has a negative ionic charge. In an embodiment of the
present invention the surfactant is a mixture of non-ionic and
anionic surfactants. The nonionic surfactant is polyoxyethylene
butylphenyl ether and the anionic surfactant is polyoxyethylene
alkylphenyl sulfate. In this embodiment, there are approximately 30
ppm of nonionic surfactant and approximately 30 ppm of anionic
surfactant in the cleaning solution. A typical concentration range
of the surfactant in the cleaning solution can be between 1-100
ppm. In an embodiment of the present invention the surfactant is an
anionic compound called MCX-SD2000 manufactured by Mitsubishi
Chemical Corporation of Tokyo Japan. MCX-SD2000 is around 1-10%
surfactant and is used in a 0.05% concentration by volume in the
cleaning solution.
The cleaning solution of the present invention is ideal for use in
a single wafer cleaning apparatus that utilizes acoustic or sonic
waves to enhance a cleaning, such as apparatus 600 shown in FIG.
6a. Single wafer cleaning apparatus 600 shown in FIG. 6a includes a
plate 602 with a plurality of acoustic or sonic transducers 604
located thereon. Plate 602 is preferably made of aluminum but can
be formed of other materials such as but not limited to stainless
steel and sapphire. The plate is preferably coated with a corrosion
resistant fluoropolymer such as Halar. The transducers 604 are
attached to the bottom surface of plate 602 by an epoxy 606. In an
embodiment of the present invention the transducers 604 cover
substantially the entire bottom surface of plate 602 as shown in
FIG. 6b and preferably cover at least 80% of plate 602. In an
alternate embodiment of the present invention there are four
transducers 604 covering the bottom surface of plate 602 in a
quadrant formation and preferably covering at least 80% of plate
602. The transducers 604 preferably generate megasonic waves in the
frequency range above 350 kHz. The specific frequency is dependent
on the thickness of the wafer and is chosen by its ability to
effectively provide megasonics to both sides of the wafer. But
there may be circumstances where other frequencies that do not do
this may be ideal for particle removal. In an embodiment of the
present invention the transducers are piezoelectric devices. The
transducers 604 create acoustic or sonic waves in a direction
perpendicular to the surface of wafer 608.
A substrate or wafer 608 is horizontally held by a wafer support
609 parallel to and spaced-apart from the top surface of plate 602.
In an embodiment of the present invention, wafer 608 is held about
3 mm above the surface of plate 602 during cleaning. In an
embodiment of the present invention, the wafer 608 is clamped face
up to wafer support 609 by a plurality of clamps 610.
Alternatively, the wafer can be supported on elastomeric pads on
posts and held in place by gravity. The wafer support 609 can
horizontally rotate or spin wafer 608 about its central axis at a
rate of between 0-6000 rpms. Additionally, in apparatus 600 wafer
608 is placed face up wherein the side of the wafer with patterns
or features such as transistors faces towards a nozzle 614 for
spraying cleaning chemicals thereon and the backside of the wafer
faces plate 602. Additionally, as shown in FIG. 6c the transducer
covered plate 602 has a substantially same shape as wafer 608 and
covers the entire surface area of wafer 608. Apparatus 600 can
include a sealable chamber 601 in which nozzle 614, wafer 608, and
plate 602 are located as shown in FIG. 6a.
In an embodiment of the present invention DI water (DI-H.sub.2O) is
fed through a feed through channel 616 of plate 602 and fills the
gap between the backside of wafer 608 and plate 602 to provide a
water filled gap 618 through which acoustic waves generated by
transducers 604 can travel to substrate 608. In an embodiment of
the present invention the feed channel 616 is slightly offset from
the center of the wafer by approximately 1 mm. The backside of the
wafer may alternately be rinsed with other solutions during this
step. In an embodiment of the present invention DI water fed
between wafer 608 and plate 602 is degassed so that cavitation is
reduced in the DI water filled gap 618 where the acoustic waves are
strongest thereby reducing potential damage to wafer 608.
DiH.sub.2O can be degassed with well known techniques at either the
point of use or back at the source, such as at facilities. In an
alternative embodiment of the present invention, instead of flowing
DiH.sub.2O through channel 616 during use, cleaning chemicals, such
as the cleaning solution of the present invention can be fed
through channel 616 to fill gap 618 to provide chemical cleaning of
the backside of wafer 608, if desired.
Additionally during use, cleaning chemicals and rinsing water such
as DiH.sub.2O are fed through a nozzle 614 to generate a spray 620
of droplets that form a liquid coating 622 on the top surface of
wafer 608 while wafer 608 is spun. In the present invention the
liquid coating 622 can be as thin as 10 micro meters. In the
present invention tanks 624 containing cleaning chemicals such as
diluted HF, de-ionized water (DI-H.sub.2O), and the cleaning
solution of the present invention are coupled to conduit 626 which
feeds nozzle 614. In an embodiment of the present invention the
diameter of conduit 626 has a reduced cross-sectional area or a
"venturi" 628, that is shown in more detail in FIG. 6d, in a line
before spray nozzle 614 at which point a gas from tank 630 that
travels through conduit 640, such as H.sub.2, is dissolved in the
cleaning solution 650 as it travels to nozzle 614. "Venturi" 628
enables a gas to be dissolved into a fluid flow 650 at gas pressure
less than the pressure of the liquid flowing through conduit 626.
The Venturi 628 creates under pressure locally because of the
increase in flow rate at the Venturi. In an alternate embodiment
gases are dissolved into the cleaning solution by a hydrophobic
contactor device 700 as shown in FIG. 7a. This contactor device 700
is put into the conduit 626. Contactor device 700 has a hydrophobic
membrane conduit 710 which allows gasses to pass through but not
water. Gas 720 is fed into membrane conduit 710 where the gas
dissolves into the liquid passing through the area 730.
Additionally, if desired, apparatus 600 can include a second nozzle
(not shown) separate from nozzle 614 for blowing N.sub.2 gas and/or
isopropyl alcohol (IPA) vapor onto the frontside of wafer 608
during rinsing and/or drying steps. An IPA vapor can be formed by
passing N.sub.2 gas through a bubbler containing IPA. Such a
process will typically produce a vapor of approximately 4% IPA in
N.sub.2. Additionally, the distance which wafer 608 is held from
plate 602 by wafer support 609 can be increased (by moving either
support 609 or plate 602) to free the backside of the wafer 608
from liquid filled gap 618 to enable the wafer to be rotated at
very high speed, such as during drying operations. Set forth below
are four embodiments of the present invention in the front end of
wafer processing where the use of the single wafer cleaning process
is particularly useful. A first embodiment is when a hydrofluoric
acid wash is used to strip the oxide surface of a wafer. A second
embodiment is when it is desired to make the surface of the wafer
hydrophobic. A third embodiment is after an O.sub.2 ashing. A
fourth embodiment is when it is desired to remove all aluminum
and/or iron contamination from the surface of the wafer. In each of
these embodiments the entire cleaning process including rinsing and
drying takes less than two minutes and the cleaning step where the
cleaning solution is used takes less than 30 seconds. In each case,
the wafer will typically include an outer silicon surface, such as
but not limited to a monocrystalline silicon substrate, an
epitaxial silicon film, and a polycrystalline silicon (polysilicon)
film. A thin oxide film, such as a sacrificial oxide or a native
oxide is typically formed on the outer silicon surface. It is to be
appreciated, however, that the cleaning process of the present
invention can be used to clean other types of wafers and
substrates, such as but not limited to gallium arsinide (GaAs)
wafers.
The first embodiment of the present invention where the use of the
single wafer cleaning tool and process is particularly useful is a
combination of using hydrofluoric acid (HF) to strip an oxide
surface of a wafer and using the modified cleaning solution as
described above to clean the wafer in less than two minutes. This
application is illustrated by a flow-chart in FIG. 8. In the first
step 800 the wafer is placed in the single wafer cleaning tool. A
substrate or wafer requiring cleaning is clamped face up to wafer
support 610. Next, the wafer is subjected to an HF step 810. During
the HF step 810 the wafer is spun at a rate between 10-2000 rpm,
and preferably 100-1000 rpm, as diluted HF is fed through nozzle
614 and sprayed onto the top surface of wafer 608 to form an HF
solution cover 622 over the entire front side of wafer 608. The HF
solution may have a concentration in the range of 5-1000 parts
water to one part HF. The HF solution is comprised of preferably 50
parts DI water to one part HF. The HF that is diluted in the HF
solution is typically purchased from the manufacturer as 49% w/w HF
to water. The wafer is exposed to the HF solution for between 20-50
seconds, and preferably 30 seconds. The wafer is exposed to the HF
solution for a time sufficient to etch either a sacrificial oxide
(typically around 50-200 .ANG.) or a native oxide (typically around
10 .ANG..) Simultaneously to feeding HF onto the top of the wafer,
water or HF is fed through feed 616 to fill the gap between the
backside of wafer 608 and plate 602 to clean the backside of the
wafer. Other solutions can be used here on the backside of the
wafer. Alternatively to HF (Hydrofluoric acid), BHF (Buffered
Hydrofluoric acid) can be used. Additionally, if desired, a voltage
can be applied to the transducers 604 to send megasonic waves
through plate 602, through water filled gap 618, through wafer 608
and into coating 622 during the HF step.
After between 20-50 seconds the flow of HF is stopped and the wafer
is exposed to a DI water rinse step as setforth in step 820. During
the DI water rinse step 820 DI water is fed through nozzle 614
while wafer 608 is rotated at between 10-1000 rpm and transducers
604 are optionally active to rinse wafer 608. The rinse temperature
is typically approximately between 19-23.degree. C., and may be
heated. During the water rinse step 820, the backside of wafer 608
can also be rinsed by flowing DI water into gap 618.
Prior to being fed through nozzle 614 the DI water rinse can be
oxygenated or ozonated at point of use by dissolving O.sub.2 or
O.sub.3 gas into the rinse water. This may be done with a venturi
device as described above (FIG. 6d) or with a membrane device as
described above (FIG. 7a). Dissolved oxygen (O.sub.2) or ozone
(O.sub.3) is added to the rinse in a concentration of greater than
1 ppm to serve as an oxidant. Alternatively H.sub.2O.sub.2 may be
added to the rinse in a concentration of greater than 100 ppm to
serve as an oxidant. Whichever oxidant is used, it should have an
oxidation potential sufficient to oxidize the most noble metal in
the solution. Copper (Cu2+), with a standard reduction potential of
0.3V, is usually the most noble metal present. Therefore a standard
reduction potential of greater than 0.5V is desired. O.sub.2 or
O.sub.3 will solvate the metal ions and prevent precipitation by
oxidizing the metal ions that are in solution. This will help
decrease the processing time by making the rinsing more effective.
The use of ozone or oxygen is also efficient and cost effective. In
an embodiment of the present invention, the DI rinse water is
degassed prior to dissolving O.sub.2 or O.sub.3 into the rinse
water.
Ozonated water can be formed by dissolving ozone (O.sub.3) in
degassed water or DI water. Ozone is generated at point-of-use from
oxygen by passing oxygen through two discharge plates. One of the
discharge plates is covered with an insulator and an alternating
current is applied to the discharge plates. The alternating current
creates small discharges between the plates that will form ozone
from the oxygen passing through the plates. The preferred
concentration of dissolved ozone is between 1 ppm and 200 ppm, and
most preferably between 2 ppm and 20 ppm. Alternatively the rinse
may be saturated with the gas. Oxygenated water is formed by
dissolving oxygen or air into deoxygenated water or DI water.
Additionally, although it is preferred to use ozonated or
oxygenated DI water during the rinsing of a wafer in a single wafer
process, one can also use ozonated or oxygenated DI water in an
immersion rinse bath of a batch type tool if desired.
In an embodiment of the present invention, prior to being fed
through nozzle 114 the rinse may also have CO.sub.2 dissolved into
it to dissipate static electricity that builds up in the rinse
water. Static electricity builds up in the rinse water because of
the rotation of the wafer between 10-1000 rpm. Without dissolved
CO.sub.2 deionized water is resistive, but with dissolved CO.sub.2
deionized water is conductive. CO.sub.2 also makes the rinse water
more acidic and therefore reduces any metallic contamination. The
CO.sub.2 can be dissolved into the rinse water with a contactor
device 700 similar to that shown in FIG. 7a. Contactor device 700
includes a conduit or plurality of conduits 710 formed from a
membrane stack 780 shown in FIG. 7b. CO.sub.2 gas 720 is fed into
the conduit 710 formed of the membrane stack 780. The contactor
device 700 also includes spaced-apart areas 730 between the conduit
710 through which DI water 650 flows. In this way, a large surface
area is achieved with CO.sub.2 gas and DI water. The membrane stack
780 in contactor device 700 is a combination of a porous polymeric
membranes 750 and a solid very thin flouropolymer sheet 740, such
as a PFA sheet as shown in a cross sectional view in FIG. 7b. The
thin solid membrane 740 prevents impurities in the CO.sub.2 gas
from dissolving into the liquid. The thicker porous membrane 750
acts as a support for the thin membrane 740. The thicker porous
membrane 750 has pores 760 on the order of 0.05 um. An example of a
suitable contactor device 700, is the Infuzor made by Pall
Corporation, Port Washington, N.Y. The polymeric membranes 740 and
750 are impermeable to liquids but permeable to gases. The membrane
stack 780 is used to prevent any impurities in the CO.sub.2 from
ending up in the rinse water. CO.sub.2 typically has organic
impurities because it is a byproduct of the petroleum industry. The
first membrane 740 is a very thin membrane that allows at least
CO.sub.2 to diffuse through, but does not allow any organic
impurities to diffuse through. In an embodiment of the current
invention, shown in FIG. 7c, the DI rinse water 650 will flow along
the thick membrane 750 and the CO.sub.2 gas 720 will flow along the
thin membrane 740. The CO.sub.2 gas 795, minus any impurities,
diffuses through the stacked membrane 780 and dissolves into the DI
rinse water 650. In an embodiment of the present invention,
CO.sub.2 is dissolved into DI water in an amount sufficient to
dissipate static electricity. In an embodiment of the present
invention, the amount of CO.sub.2 dissolved into the DI water is
sufficient to decrease the resistivity of the DI water to less than
5 Megaohm.cm. The CO.sub.2 may also be dissolved into the rinse
water using a venturi device as described above.
The rinse may also have isopropyl alcohol (IPA), or any other
liquid with a surface tension lower than that of water, added to
it. IPA aids by making the rinse spread out over the surface of the
wafer so that the chemicals are removed more quickly. The IPA also
helps the rinse spin off of the wafer during spinning.
Alternatively, IPA vapor can be blown onto the wafer frontside by a
second separate nozzle while rinsing to assist the rinse. The DI
water rinse step is meant to remove the chemical from the etching
and/or cleaning step and to replace these chemicals with pure DI
water. The removal of chemicals from the wafer happens through a
combination of convection and diffusion. Closer to the wafer
surface, chemicals are removed by the rinsing DI water by diffusion
only. The diffusion rate of chemicals close to the wafer surface is
dependent upon the boundary layer thickness. The boundary layer
thickness can be made small by spinning the wafer at high rotation
rates. In an embodiment of the present invention, IPA vapor is
directed at the wafer surface. This IPA vapor reduces the boundary
layer and pushes the remaining chemicals and DI water away from the
surface. This is an very efficient way of shortening the rinse.
Additionally, if desired, megasonic energy can be applied while
rinsing the wafer in step 820.
After a rinsing sufficient to remove all HF and to stop the etching
of the oxide surface (usually between 10-50 seconds, and preferably
about 20 seconds) the flow of DI water is stopped. The rinse step
is efficient because the centrifugal force created by spinning the
wafer helps to quickly remove the rinse.
Next, as set forth in step 830, the wafers are cleaned with the
cleaning solution of the present invention. The cleaning solution
of the present invention comprising ammonium hydroxide
(NH.sub.4OH), hydrogen peroxide (H.sub.2O.sub.2), water (H.sub.2O),
a chelating agent such as those discussed above, and a surfactant
such as those discussed above, is sprayed by nozzle 614 onto the
top surface of wafer 608 in the chemical cleaning step 830. In an
embodiment the temperature of the cleaning solution is between
40-85.degree. C. At this time, the back-side of the wafer is also
cleaned with the cleaning solution, or alternately with another
solution, such as DI water. While the cleaning solution is sprayed
onto the wafer 608, the wafer 608 is rotated at a rate between
10-200 rpm to form a thin coating 622 of cleaning solution over the
top of wafer 608. Wafer 608 is exposed to the cleaning solution for
between 30 and 60 seconds and preferably for a time less than 90
seconds. The transducers 604 produce acoustic waves while flowing
the cleaning solution of the present invention onto wafer 608. The
transducers 604 produce acoustic waves that travel through plate
602, through liquid filled gap 618, and through wafer 608 and into
cleaning solution coating 622 on wafer 608 to enhance the cleaning
of wafer 608. It is to be appreciated that the megasonic waves
entering the water filled gap 618 also help to clean the backside
of wafer 608.
In an embodiment of the current invention a combination of the
cleaning solution and megasonics will allow for a dramatic particle
and metal contaminant removal. In this embodiment the wafer is
exposed to the cleaning solution for approximately 30 seconds and
megasonics are applied to the wafer. Before the cleaning step the
surface of the wafer can have greater than 1000 contaminant
particles that are each larger than 0.1 .mu.m in size. After the
cleaning step in this embodiment, the surface of the wafer can have
less than 50 contaminant particles that are each larger than 0.1
.mu.m in size. In this embodiment the wafer surface can start out
with greater than 1.times.10.sup.11 metallic atoms/cm.sup.2 before
the cleaning step, and after the cleaning step the wafer surface
can end with less than 1.times.10.sup.10 metallic atoms/cm.sup.2
(excluding aluminum atoms) on the surface of the wafer after the
cleaning step.
In an embodiment of the cleaning process of the present invention
H.sub.2 gas is dissolved into the cleaning solution while the
cleaning solution is fed through conduit 126. H.sub.2 gas is
dissolved into the cleaning solution to provide cavitation (bubble
creation) to the cleaning solution coating 122 on the wafer 108.
Providing cavitation to the cleaning solution of the present
invention enhances the cleaning of the top surface of wafer 108.
Although H.sub.2 is the preferred gas other suitable cavitation
gases such as nitrogen (N.sub.2), helium (He), Argon (Ar) or oxygen
(O.sub.2) can also be used. Dissolving a gas into the cleaning
solution accelerates cleaning processes that utilize acoustic or
sonic waves to enhance cleaning. Dissolving gas molecules into the
cleaning solution makes acoustic enhancement more efficient by
improving the cavitation behavior of the cleaning solution. In the
present invention between 0.01 to 20 mg/l of H.sub.2 is dissolved
into the cleaning solution, and most preferably about 0.1 to 5 mg/l
of H.sub.2. Alternatively, between 1 and 20 mg/l of O.sub.2 can be
dissolved into the cleaning solution.
After sufficient cleaning of wafer 608 in step 830 the flow of the
cleaning solution of the present invention is stopped and the wafer
rinsed once again with DI water as set forth in rinse step 840.
O.sub.2 or O.sub.3 dissolved in the rinse is especially useful in
guaranteeing that all chelating agents and surfactants are removed
during this rinse step. O.sub.2 or O.sub.3, as well as
H.sub.2O.sub.2, may be added to the rinse as described above to
serve as an oxidant. CO.sub.2 and isopropyl alcohol (IPA) may also
be dissolved in this rinse as described above. The backside of the
wafer can also be rinsed at this time by flowing DiH.sub.2O into
gap 618. The wafer is rinsed for around 20 seconds or more. The
rinsing step after the cleaning solution is meant to remove all the
chemicals from the wafer surface, i.e. ammonium hydroxide, hydrogen
peroxide, the chelating agent, and surfactants.
Next, as set forth in step 850, the wafer is dried. The wafer is
then dried by spinning at very high rotation speeds between
100-6000 rpm, preferably around 3000 rpm, for about 20 seconds and
using the air flow around the wafer to dry the wafer. If desired,
N.sub.2 and/or IPA vapor may be blown on the wafer to assist in
drying the wafer. Typically, the rotation rate of the wafer during
the drying step is greater than the rotation rate of the wafer
during the rinse step. After drying, the wafer is then removed from
the single wafer cleaning tool. The above described process of the
present invention is able to complete a full cleaning cycle
including HF etching, rinsing, chemical cleaning, rinsing, and
drying in less than three minutes and preferably in less than two
minutes as illustrated in FIG. 1b. The clean wafer can then undergo
a high temperature thermal process step, typically greater than
400.degree. C., such as but not limited to a gate oxidation step, a
chemical vapor deposition (CVD) step, or an anneal step.
The second embodiment of the current invention, shown in FIG. 9 in
flow chart form, is a process which can be used to make the surface
of the wafer hydrophobic. Instead of performing the HF step and
then the chemical cleaning step, as set forth in FIG. 8, this
process places the cleaning step before the HF step. Performing the
HF step after the cleaning step leaves the surface of the wafer
hydrophobic at the end of the cleaning process. All of the same
details as described above with respect to FIG. 8 apply to the
respective steps in the current process. The wafer is first placed
in the single wafer cleaning tool by clamping the wafer face up to
wafer support 610 (step 900.) The wafer is then spun as described
above. The cleaning solution of the present invention, that
optionally contains a cavitation gas, is then fed through nozzle
614 onto the top surface of wafer 608 as set forth in chemical
cleaning step 910. Next, the wafer is rinsed in step 920 with a
rinse that optionally contains an oxidant and/or CO.sub.2 as
described above. Megasonics may also be applied during the rinse.
Once rinsed, a HF solution is dispensed onto the wafer in HF step
930. This solution will strip the silicon dioxide layer 1000 (FIG.
10a) from the pure silicon surface 1010. After the HF step 930, the
pure silicon surface looks like FIG. 10b. The silicon surface 1010
in FIG. 10b is hydrophobic because of the hydrogen and fluorine
termination groups. After the HF step 930 there is an optional
rinse step 940. A hydrophobic surface, e.g., is useful when either
a cobalt sputter or gate oxidation is done after the cleaning
process. After the last wet step (either the HF step 930 or the
rinse step 940) the wafer is dried, in drying step 950, in a manner
set forth above. The wafer is then removed from the single wafer
cleaning tool and processed in another single wafer tool, such as,
for example, a cobalt deposition chamber or a gate oxidation
chamber.
A third embodiment of the present invention, as shown in the flow
chart in FIG. 11, is the use of the single wafer cleaning tool
after an O.sub.2 ashing of the wafer to remove a photoresist. Most
O.sub.2 plasma ashing steps are carried out in a single wafer mode
and it is therefore very useful to have a single wafer cleaning
method instead of a batch cleaning method after a O.sub.2 plasma
ashing step. All of the same details as described above apply to
the respective steps in the current process. The most significant
difference in this process compared to the others disclosed in the
present invention is that there is no HF step. There is no HF step
is because the oxide surface on the wafer is needed in subsequent
processing. In this application, after the O.sub.2 ashing step
1100, a wafer having an outer oxide film on a silicon surface is
placed in the single wafer cleaning tool face up as described above
and the wafer is spun. Next, as set forth in chemical cleaning step
1110, the wafer is cleaned with the cleaning solution of the
present invention as described above. Megasonic can be applied to
the wafer to aid in the cleaning of the wafer. Before dispensation,
the cleaning solution may optionally have a cavitation gas
dissolved into it. The cleaning solution is then dispensed onto the
wafer as the wafer is rotated in step 1110. The cleaning solution
removes the ash residue left by the ashing step and removes most
metals and chlorine from the surface of the wafer that the ashing
does not remove. In this embodiment the cleaning step may exceed 30
seconds. With the elimination of the HF step the cleaning step 1110
may be longer and the entire cleaning process may still be done
within two minutes. After the cleaning step, the wafer is rinsed in
step 1120 with a rinse solution optionally containing an oxidant
and/or CO.sub.2 as described above. Next, the wafer is dried in
step 1130 by spinning at high speeds as described above. After this
cleaning process, where an oxide layer is left on the surface of
the wafer, any application where the silicon of the wafer needs to
be protected, such as ion implantation, is suitable.
In the fourth embodiment of the present invention the single wafer
cleaning tool is used to remove all aluminum and iron contamination
from the surface of the wafer. An embodiment of this invention is
shown in the flow charts of FIG. 12. This embodiment uses a very
short HF step that etches away only about 0.5-5 .ANG. of a thermal
oxide on the wafer silicon surface. This quick etching in
combination with the cleaning step will quickly remove all aluminum
and iron contamination, as well as any other contaminants, from the
surface of the wafer within approximately 30-40 seconds. Without
the short HF step, the cleaning solution alone would take
approximately 10 minutes to remove all of the aluminum and iron.
The wafer can be contaminated with around 2.times.10.sup.11
atoms/cm.sup.2 of aluminum ions after being in an ion implanter or
in an etching chamber. The present cleaning application will reduce
that concentration of aluminum and iron atoms to around
1-5.times.10.sup.10 atoms/cm.sup.2. In the embodiment shown in FIG.
12, the short HF step 1230 is immediately before the cleaning step
1240 and there is no rinse between the HF step 1230 and the
cleaning step 1240.
In this embodiment to remove all aluminum contamination from the
surface of wafer, the wafer is first placed in a single wafer
cleaning tool in step 1200 after being contaminated with aluminum
and iron in, for example, either an ion implant chamber or an
etching chamber. At this point, as shown in FIG. 13a, there is a
thin oxide layer 1300 on the surface of the silicon wafer 1310.
Once loaded into the single wafer cleaning tool, the wafer is spun
and optionally rinsed. As set forth in step 1210, the optional
rinse may optionally contain an oxidant and/or CO.sub.2 as
discussed above. Megasonics may also be applied during this
optional rinse. If no initial rinse is used, then the wafer after
being loaded into the single wafer machine is spun and HF is
dispensed on the spinning wafer for less than five seconds and
preferably for 2-3 seconds as set forth in step 1230. If an initial
rinse is used, HF is dispensed for 2-3 seconds on top of the rinse
water on top of the spinning wafer. The cleaning solution of the
present invention is then immediately dispensed on top of the HF
solution on top of the wafer in chemical cleaning step 1240 to
produce an HF covered wafer. The cleaning solution consists of a
mixture of ammonium hydroxide, hydrogen peroxide in water with a
chelating agent added. Additionally, a surfactant may be added as
discussed above. The cleaning solution quickly neutralizes the HF
solution and stops the etching. Because the cleaning step quickly
neutralizes the HF solution, the oxide film 1300 is only slightly
etched, and an oxide 1320, as shown in FIG. 13b, which is only
0.05-5 .ANG. thinner remains after the cleaning step in step 1240.
Because of the quick neutralization and the elimination of a rinse
step, an HF step followed immediately by a cleaning step increases
the efficiency of the HF etch quenching. A cavitation gas may
optionally be dissolved into the cleaning solution before it is
dispensed on the wafer. After the cleaning step 1240 the wafer is
rinsed in step 1250 as described above. After the wafer is
sufficiently rinsed it is dried in drying step 1260 by spinning the
wafer at high speed as set forth above. The wafer is then removed
from the chamber and thermally processed in a single wafer furnace.
When using a single wafer furnace, it is very useful to have a
single wafer cleaning method instead of a batch cleaning method.
The thermal processes are typically performed at temperatures
exceeding 400.degree. C. The thermal process may be an anneal, a
chemical vapor deposition (CVD), or an oxidation. All metals must
be removed from the surface of the wafer before any thermal
processing steps because the metals will become embedded in the
wafer during the thermal processing.
It is to be appreciated that although the cleaning process of the
present invention is ideally carried out in an apparatus 600 as
shown in FIG. 6a the cleaning process of the present invention can
utilize other cleaning apparatuses. For example, the acoustic
energy need not necessarily be applied from the bottom of the wafer
but can also be applied to the front side. Additionally, the
acoustic devices need not necessarily cover the entire surface area
of the wafer 600 but may only cover a portion if desired. Still,
the acoustic energy can be applied directly to nozzle 614 so that
the droplets contained by nozzle 614 contain acoustic waves. In
fact, although preferred acoustic energy is not required during
cleaning. Similarly, the cleaning solution of the present invention
need not necessarily be sprayed onto top surface of the wafer but
can also be dispensed onto the wafer by a constant stream of
liquid. Furthermore, the cleaning solution can also be supplied at
the same time onto the front of the wafer and the backside of the
wafer as well as onto the edge. Although ideally situated for a
single wafer process, a solution comprising NH.sub.4OH,
H.sub.2O.sub.2, H.sub.2O, a chelating agent, and a surfactant,
according to the present invention can also be used in an immersion
bath for a batch type cleaning process and provide improved
cleaning. It is to be appreciated that the disclosed specific
embodiments of the present invention are only illustrative of the
present invention and one of ordinary skill in the art will
appreciate the ability to substitute features or to eliminate
disclosed features. As such, the scope of applicant's cleaning
solution and cleaning methodology are to be measured by the
appended claims that follow.
Thus, a novel cleaning method and solution for use in a single
wafer cleaning process have been described.
* * * * *
References